Many commercial products contain components which exercise a beneficial effect for only a limited time after introduction into their intended environment, being rapidly consumed, metabolized, vaporized or otherwise lost. To have continued effectiveness, such products must be reapplied at intervals, providing an undesirable and perhaps harmful excess at the times of reapplication and barely adequate levels at later times.
Microencapsulation techniques address the problem of controlled release by enclosing the transient component within hollow shells of differing size and wall thickness, which dissolve or otherwise rupture at different intervals to provide a more or less steady supply.
The temporary shells of microencapsulation can be replaced by more permanent semipermeable shells which allow escape through the shell wall without shell destruction, or the entire microcapsule replaced by a homogeneous semipermeable vehicle containing the active ingredient as a pure impregnant, solute or precipitate. In this latter process, the host vehicle serves not to enclose the active ingredient within a wall, but as a carrier from which it can only slowly escape by solution, diffusion, evaporation or some other rate-limited process. The utility of a particular host material as such a carrier depends on such properties as liquid content, pore size, compatibility with various environments, surface energy and wettability, susceptibility to post-impregnation modifications in properties, and ease of manufacture in suitable physical forms. The commercial exploitation of slow release carrier vehicles requires the availability of inert, microporous materials which are readily impregnable with a wide variety of substances, have controllable porosity, and possess acceptable physical properties.